Many nuclear power plants are pressurized water reactors.
These nuclear power plants comprise at least two mutually independent water circuits: the primary circuit and the secondary circuit.
The function of the primary circuit is the capture of heat from the nuclear reaction. Heat is produced in very large quantities by the fission of uranium atoms.
Placed within the reactor vessel, water in the primary circuit thus achieves a temperature of approximately 320°, after having been heated during the chain reaction.
This water does not boil, as it is very highly pressurized. It is then routed from the reactor core to the primary circuit, which is a closed circuit.
The water in the primary circuit heats the water in the secondary circuit via a steam generator, which permits the exchange of heat between the two independent circuits. The pipes in the primary circuit heat the water in the secondary circuit by contact to form water vapor.
The water in the secondary circuit is at a lower pressure, and is therefore converted into steam. This steam drives the reactor turbine. The rotation of the turbine in turn drives the generator, thus permitting the production of electricity.
Other nuclear power plants have a primary circuit only, and the water which is heated by the nuclear reaction is vaporized to form water vapor.
It is surprising to observe that the generation of electricity is more efficient where a proportion of the heat generated by the power plant is used, not in the steam turbine of the power plant, but for the preheating of a gas which is to be fed to a turbine.
According to the invention, primary (rather than residual) thermal energy is used, which is normally employed for the generation of electricity in the power plant, at the time, moreover, where electricity is to be supplied to the grid system. The superior efficiency of the vaporized air (or atmospheric gas) cycle, in comparison with the steam cycle of the steam turbine of the power plant, is exploited to deliver more energy to the grid system.
During periods of low electricity consumption, it is sometimes necessary to store thermal energy generated by the power plant. Thermal energy storage facilities required for this purpose are voluminous, expensive and relatively difficult to implement.
The present invention proposes the elimination or reduction in size of these storage facilities, and the replacement thereof, at least partially, by a system for the liquefaction of air or of atmospheric gas.
US-A-2012151961 describes a method for the storage of liquefied air. During phases of low electricity demand, air is liquefied and stored. During phases of high electricity demand, liquid air is vaporized in a system which optimizes the recovery of cold, to generate a pressurized fluid which drives a turbine for the production of electricity. The energy obtained (and consequently the efficiency of storage) is all the more efficient if the fluid is heated using residual heat prior to expansion.
The article “Cryogenic Solutions for Energy Storage and the Optimization of Energy Supply” in the Revue Générale du Froid, by Dubettier et al, describes the heating of vaporized air using residual heat, or by means of natural gas burners to increase the energy produced by the expansion of air.
The solution described in the prior art is as follows:                during phases of low demand:                    Electrical energy is used to produce liquid air            A proportion of the available thermal energy is stored for use at times of high demand, and will be used to heat up pressurized gas prior to expansion                        and during phases of high demand:                    Liquefied gas is vaporized, with the recovery of cold, to produce a pressurized gas            The pressurized gas is heated using previously stored thermal energy            The gas is expanded to produce electricity                        